1. Field of the Invention
The present invention relates to a combined externally pressurized gas and magnetic bearing assembly in which an externally pressurized gas bearing and a magnetic bearing are combined together, and also to a spindle device employing the combined externally pressurized gas and magnetic bearing assembly. More particularly, the present invention relates to the combined externally pressurized gas and magnetic bearing assembly and the spindle device both suited for use in, for example, a high speed milling machine.
2. Description of the Prior Art
The magnetic bearing is known to have a feature in that because it has a relatively large bearing gap a loss of torque during rotation thereof is extremely small and it can exhibit a high static rigidity by an integral control.
FIG. 28 illustrates a longitudinal sectional view showing a prior art spindle device utilizing a magnetic bearing, which is used in a high-speed aluminum milling machine. This prior art spindle device includes a touch-down bearing 251, a tool 252, a displacement sensor 253, a radial magnetic bearing 254, an axial magnetic bearing 255, a drive motor 256, a radial magnetic bearing 257, a displacement sensor 258, and a main shaft 259. The spindle device utilizing the magnetic bearings has a performance of a maximum rotation of 40,000 rpm, and a maximum output of 15 kW, a maximum machining capacity of 1,250 cm.sup.3 /min, thus exhibiting an excellent performance for an aluminum milling work.
However, the spindle device utilizing the magnetic bearing is susceptible to influence brought about by a natural frequency of bending of the main shaft during the milling operation and, for this reason, requires the use of an extremely complicated control system. Accordingly, the known spindle device of the type discussed above is not suited as a spindle device for a versatile machine tool that is required to accommodate various processing conditions.
On the other hand, a non-contact bearing currently available other than the magnetic bearing includes an externally pressurized gas bearing. Although the externally pressurized gas bearing is known to have a high rotational accuracy and an excellent dynamic stability, the externally pressurized gas bearing has been little used in the versatile machine tool because of the compressivity and, hence, a low static rigidity and a low load bearing capacity.
In view of the foregoing, attempts have recently been made to use, as a spindle device for high speed machining purpose, a spindle device utilizing a composite bearing in which the externally pressurized gas bearing and the magnetic bearing are combined, such as shown in FIG. 29 in a longitudinal sectional representation. Referring to FIG. 29, reference numeral 263 represents a displacement sensor; reference numeral 264 represents a radial magnetic bearing; reference numeral 265 represents an axial magnetic bearing; reference numeral 266 represents a drive motor; reference numeral 267 represents a radial magnetic bearing; reference numeral 268 represents a displacement sensor; reference numeral 270 represents a displacement sensor; reference numeral 271 represents a main shaft; and reference numerals 272 and 273 represent respective externally pressurized gas bearings.
However, the spindle device utilizing the composite bearing shown in FIG. 29 has a problem in that since the magnetic bearings 264 and 267 and the externally pressurized gas bearings 272 and 273 are disposed one after another in an axial direction of the main shaft 271, not only is a relatively long main shaft 271 required, but the natural frequency of bending tends to be lowered. Also, since the spindle device shown in FIG. 29 makes use of a control system of a structure similar to that required in the spindle device utilizing solely the magnetic bearings, an additional problem has arisen that the dynamic stability of the externally pressurized gas bearing is impaired and it tends to function as a source of external disturbances.
Accordingly, the art has not yet been developed to the extent as to fulfill the objective of utilizing advantages of the externally pressurized gas bearing and also those of the magnetic bearing while counterbalancing demerits of those respective bearings.